Repository logo

Exploring the onset of B12-based mutualisms using a recently evolved Chlamydomonas auxotroph and B12-producing bacteria

Accepted version

Change log


Bunbury, Freddy 
Deery, Evelyne 
Sayer, Andrew 
Bhardwaj, Vaibhav 
Harrison, Ellen 


Cobalamin (vitamin B12) is a cofactor for essential metabolic reactions in multiple eukaryotic taxa, including major primary producers such as algae, and yet only prokaryotes can produce it. Many bacteria can colonize the algal phycosphere, forming stable communities that gain preferential access to photosynthate and in return provide compounds such as B12. Extended coexistence can then drive gene loss, leading to greater algal–bacterial interdependence. In this study, we investigate how a recently evolved B12-dependent strain of Chlamydomonas reinhardtii, metE7, forms a mutualism with certain bacteria, including the rhizobium Mesorhizobium loti and even a strain of the gut bacterium E. coli engineered to produce cobalamin. Although metE7 was supported by B12 producers, its growth in co-culture was slower than the B12-independent wild-type, suggesting that high bacterial B12 provision may be necessary to favour B12 auxotrophs and their evolution. Moreover, we found that an E. coli strain that releases more B12 makes a better mutualistic partner, and although this trait may be more costly in isolation, greater B12 release provided an advantage in co-cultures. We hypothesize that, given the right conditions, bacteria that release more B12 may be selected for, particularly if they form close interactions with B12-dependent algae.



Journal Title

Environmental Microbiology

Conference Name

Journal ISSN


Volume Title


Society for Applied Microbiology
Biotechnology and Biological Sciences Research Council (BB/I013164/1)
European Space Agency (ESA) (via MELiSSA Foundation) (SCK CEN: CO-90-16-4078-02)
Biotechnology and Biological Sciences Research Council (BB/M011194/1)
Biotechnology and Biological Sciences Research Council (BB/S014020/1)
BBSRC (via University of Kent) (210 22986)